Where is hydrogen peroxide found in the body?

According to Discover Magazine, hydrogen peroxide is found naturally in the human thyroid, gut and lungs. Science Daily states that the compound is produced by the body to keep its cells healthy.

Scientists have discovered that when the immune system is activated, hydrogen peroxide arrives at the site of infection a great deal earlier than white blood cells. Discover Magazine states that hydrogen peroxide has some role in calling the white blood cells to a damaged area. Further, there is speculation that increased production of hydrogen peroxide may have to do with diseases marked by high levels of white blood cells.